Well folks, after 60-odd years of commercial television in the United States, there is a law on the books, which aims to regulate loud commercials. This issue, the perceived loudness of commercials versus the program material they are played alongside, has been with us for as long as television has been with us.

There was considerable attention given to commercial loudness in the 1960s, although regulators could never entirely define the problem, let alone the solution. Part of the problem was that loudness, as perceived by the human hearing system, does not necessarily relate well to the objective indications of the type of audio level meters traditionally used in television production and broadcasting.

In this country, we have traditionally used the standard volume indicator, colloquially known as the VU meter, for program mixing and monitoring. The time constants and ballistics of the VU meter allow an experienced mixer to do an artful mixing job, and they work very well for setting levels with sine-wave tones.

But peak amplitude is not the whole story; we know that the audio waveform's peak-to-average ratio, along with frequency content and other factors, has a great influence on its perceived loudness. If a VU meter is very dynamically bouncing up and down between 0 VU and the bottom portion of the scale, the sound's perceived loudness can be significantly lower than if the same material is dynamically compressed such that the needle, or virtual needle, hangs constantly at 0 VU, although both situations might be described as "zero" levels.

The situation with quasi-peak meters such as PPMs is even worse, as their fallback times are much slower, causing them to naturally hang at the upper levels far longer. The work in the 1960s produced the CBS loudness meter, which takes some of these effects into account. It worked pretty well, actually, but it never was codified into any law or regulation.

CALM ACT

The Commercial Advertisement Loudness Mitigation (CALM) Act was passed by Congress and signed into law last December, and is now known as Public Law 111-311.

At the time of this writing, the text of the final version of the bill as passed by the Senate and House was unavailable online, so the following description is drawn from the text of H.R. 1084 as considered in the Senate, dated Dec. 16, 2009, plus updates on changes as supplied by various press releases.

The FCC, by the way, has a page addressing commercial loudness that, at the time of this writing, contains nothing about the new law. It tells us that the volume buttons on our remote controls are a good way to control loudness.

What does this law actually require, anyway? Within one year after the date of enactment of Public Law 111-311, that is, by the middle of December, 2011, the FCC is required to prescribe a regulation that is limited to incorporating by reference and making mandatory, subject to some waivers, ATSC A/85, ATSC Recommended Practice: Techniques for Establishing and Maintaining Audio Loudness for Digital Television, plus any successors to that document, "…only insofar as such recommended practice concerns the transmission of commercial advertisements…"

The FCC regulation that is developed pursuant to this law will become effective one year after the date of its adoption. So it could be December 2012, before the regulation goes into effect. And there is the power vested in the FCC to waive the regulation for one to two years for any broadcaster, cable operator or other multichannel programming distributor who demonstrates that obtaining the equipment to comply with the FCC regulation would be a financial hardship.

That's the law. It is about three pages in length, and it really leaves the question of exactly what TV stations and other program distributors are required to do, and what parts of ATSC A/85 will be required to be applied to their operations, to the FCC. The entire A/85 document is some 70 pages long, with annexes beginning at page 37.

There is a wealth of information in the document about everything having to do with DTV audio and with ancillary topics like room acoustics. Probably the portion most germane to the apparent intent of the law is Chapter 8, "Methods to Effectively Control Program-to-Interstitial Loudness," which describes two methods of control using dialnorm.

IMPORTANT CONCEPTS

We should pause here to note two important concepts addressed in A/85. First, it specifies the Anchor Element, the reference point around which other audio elements are balanced in producing the audio mix, or on which a viewer would focus when setting the volume control, as the dialog.

Second, it introduces the concept of loudness measurement over a specified time interval, as opposed to the more traditional practice of instantaneous audio level measurement, and specifies that loudness should be measured using the method and algorithm described in ITU-R BS.1770.

BS.1770 specifies measuring loudness over certain time intervals in units of dB LKFS, which are dB relative to Full Scale using K-weighting, the weighting algorithm described in BS.1770.

Dialnorm is a number embedded in the AC-3 metadata, which is equal to the absolute value of the dialog level, which, in turn, is the loudness, in LKFS units, of the anchor element. Dialnorm effectively operates an automatic volume control in the DTV receiver.

When the AC-3 decoder in a receiver is in its default state, receiving no dialnorm number, it attenuates the decoder's audio output gain to 31 dB below full-scale, or –31 dBFS. This may be thought of as the "static reference audio output level."

The dialnorm number is a positive number between 1 and 31, and it dynamically specifies the amount of attenuation to be applied to the audio output signal relative to 0 dBFS. For example, if the dialnorm number is 31, full attenuation is applied. If the dialnorm number is 24, 7 dB less attenuation will be applied, resulting in an output reference level of –24 dBFS. Thus, the lower the number, the higher the output volume.

The two ways described in Chapter 8 of A/85 to normalize the loudness of the various audio segments in the program stream are the fixed dialnorm system and the agile dialnorm system.

If our target loudness level were –24 dB LKFS, for example, in the fixed dialnorm system, we would normalize the loudness of all audio segments to this level, and continuously transmit the dialnorm number 24. In this scenario, the DTV receiver's AC-3 decoder would adjust the automatic volume control to the dialnorm number 24, and the control would stay at that setting.

In the agile dialnorm system, all the various segments retain their native, varying, loudness levels, and their native dialnorm numbers are transmitted, causing the automatic volume control in the DTV receiver to continuously adjust based on the incoming dialnorm numbers. Either approach results in the delivery of relatively stable loudness to the viewer.

This would be your author's interpretation of the spirit of the new law. What is actually promulgated by the FCC remains to be seen.

Randy Hoffner is a veteran of the big three TV networks. He can be reached through TV Technology.

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